The present invention generally relates to cardiac external medical devices.
A treatment known as Cardiac Potentiation Therapy (CPT) can be used to increase a person's cardiac output by electrically stimulating one or more heart chambers to induce post-extrasystolic potentiation. This therapy is used to improve hemodynamic function for a patient that has lost function due to myocardial infarction. In brief, CPT works by increasing the intracellular calcium in cardiomyocytes, thereby increasing the force of myocardial contraction in response to stimulation. Such benefit is achieved when the electrical stimulus is delivered to the heart near the end of the refractory period of the cardiac cycle. Generally, the electrical stimulus delivered does not induce a cardiac contraction, but rather facilitates calcium transport into the cell, which, in turn, increases the strength of a subsequent contraction. In this way, CPT increases the patient's ejection fraction, which, in turn, increases cardiac output.
As is known, proper application of CPT requires knowledge of the cardiac refractory period. A stimulus provided before the end of the refractory period has been found to not induce a cardiac action potential and tends to not affect contractile force. Conversely, a stimulus provided too long after the end of the refractory period tends to have a reduced benefit. In the extreme, for example, if the stimulus is provided at the regular R-R interval, no effect is generally realized.
Some conventional methods used for determining the end of the refractory period have involved using one or more signals stemming from inside the body. For example, an intra-cardiac electrocardiogram (ECG) signal can be used to determine the end of the refractory period by adjusting the application of the electrical stimulus, or rather, the potentiation pulse. If the potentiation pulse produces a cardiac action potential, then such pulse is known to be outside the refractory period. Conversely, if no action potential is produced following delivery of the potentiation pulse, then such pulse is known to be inside the refractory period. The refractory period duration doesn't change very much on a beat-to-beat basis. As such, a “hunting” algorithm can be employed, for example, to continually adjust the location of the potentiation pulse, e.g., relative to the R wave.
Invasive blood pressure waveforms can alternatively be used in adjusting the location of the potentiation pulse. For example, an optimal location of the potentiation pulse is the location that produces the maximum diastolic blood pressure on the next subsequent beat. Therefore, if a hunting algorithm is employed, the best relative location of the potentiation pulse can be determined by measuring the diastolic blood pressure over a series of cardiac cycles.
To date, implantable cardiac medical devices have generally been used in providing CPT to patients. Some examples of implantable cardiac medical devices include hemodynamic monitors (IHMs), cardioverter-defibrillators (ICDs), cardiac pacemakers, cardiac resynchronization therapy (CRT) pacing devices, and drug delivery devices. Implantable devices have generally been viewed as ideal systems for administering CPT to patients due to a number of reasons. First, the devices are already configured for internally monitoring the hemodynamics of a patient, e.g., via sensors from the implantable devices. As such, the hemodynamic information gathered is generally quite precise. As a result, providing CPT to a patient based on such information is both effective and efficient. Second, many implantable devices are already equipped to deliver electrical stimulus to the patient when the need arises. Therefore, these devices can further be used to deliver CPT, as described above, via programming of the devices.
It is well known that patients not having implantable medical devices can be just as susceptible to reduced cardiac output episodes as those patients having such devices. In turn, such patients would equally benefit from CPT; however, the patients would only have access to CPT through external cardiac medical devices. For example, patients suffering from Pulseless Electrical Activity (PEA) may not be implanted with cardiac medical devices. As is known, PEA is a condition that involves a reduced (or nonexistent) cardiac output, whereby a person in PEA has an organized electrical cardiac rhythm but little or no blood flow. PEA can commonly occur for a period of time after a patient has been given defibrillation shocks from an external cardiac medical device. For example, in “successful” defibrillations, where patients regain an electrical pulse as demonstrated by an ECG, approximately 50% of the patients will not exhibit a physical pulse as demonstrated by restored, peripherally measured, blood pressure. In such cases, CPT could be used to help restore normal cardiac output during the recovery period of the patient. As is known, PEA is also found to occur at other times as well. Currently, the effects of therapy are limited in treating PEA; however, as alluded to above, CPT may be beneficial.
Unfortunately, no external cardiac medical devices are commercially available which have been configured to provide CPT. One reason for this could be that implementation of CPT with respect to external devices presents a number of challenges not currently faced with implantable devices. For example, as known from CPT-equipped implantable devices, effective administration of CPT depends greatly on accurate monitoring of a patient's cardiac signals and/or hemodynamics; however, such accurate monitoring can be problematic for external devices. For instance, ECG signals measured externally can be difficult to monitor because noise from the external stimulation pulse often obscures the cardiac action potential.
Another reason for the lack of CPT-equipped external devices may be that the design of implantable devices with CPT functionality is still fairly new. As such, the implementation of CPT with implantable devices may be indirectly delaying implementation of CPT with external devices. A further reason may be that testing the populations of patients requiring CPT when using external devices has proved to be more difficult in comparison to testing patients with implantable devices, from which information can simply be downloaded and studied.
What are needed are apparatus and systematic methods to address or overcome one or more of the challenges briefly described above with respect to external cardiac medical devices.